PDP (Plasma Display Panel) equipment using a plasma display panel and LCD (Liquid Crystal Display) equipment using a liquid crystal display panel as thin and lightweight display equipment have come to attract public attention. Such display equipment is conventionally of the direct-drive type using a digital video signal, wherein a contour emphasizing circuit, such as shown in FIG. 1, is used for obtaining a contour-emphasized video signal from the input video signal.
The contour emphasizing circuit shown in FIG. 1 comprises a contour pick-up unit 10, a delay adjusting unit 12 and adder 14.
The contour pick-up unit 10 comprises one-dot delay units 18, 20 for sequentially delaying by one-dot the digital luminance signals (an example of a video signal) which have been input to the input terminal 16, an adder 22 for adding the luminance signal Y input to the input terminal 16 to the output signal from the one-dot terminal 20 to obtain the sum, a multiplier 24 for multiplying the sum by coefficient ¼ for outputting the product thereof, a multiplier 26 for multiplying the output signal from the one-dot delay unit 18 by coefficient ½ for outputting the product thereof, and a subtracter 28 for subtracting the output signal of the multiplier 24 from the output signal of the multiplier 26; wherein the contour component (i.e., high-pass component) HE in a horizontal direction of the reference picture element are sampled, for output, from the picture elements on the left side and right side (preceding and subsequent picture elements on a time basis) of the reference picture element.
A delay adjusting unit 12 is designed to adjust the timing of the luminance signal, as provided to the input terminal 16 and passed on to the adder 14, to the timing of the contour component HE as sampled by contour pick-up unit 10 and passed on to the adder 14. The delay adjusting unit 12 coordinates the timing of the luminance signal Y to the contour component HE by delaying the output of luminance signal Y, which has been input to the input terminal 16, for a predetermined time interval.
The adder 14 adds the luminance signal Y, which is output from the delay adjusting unit 12, to the contour component HE, which is sampled by the contour pick-up unit 10, to generate the sum (Y+HE) representing a contour-emphasized luminance signal.
However, the contour emphasizing circuit shown in FIG. 1 suffers from a problem, as is further described below, in that its design allows the contour component HE sampled by the contour pick-up unit 10 to be directly output to the adder 14 regardless of whether a high or low level of luminance signal Y is input to the input terminal 16.
As a result of the above problem, an unnatural picture having overemphasized contour can be produced if a contour component having too high a value is added to a relatively dark picture of a low luminance level. Another problem of the circuit is that the contour cannot be emphasized sufficiently if a contour component having to low a value is added to a bright picture having a high luminance level.
The present invention is designed for the purposes of solving the above problems and for providing a contour emphasizing circuit capable of accomplishing a level of contour emphasis that matches the luminance level of an input video signal.